Mixer stage of a superheterodyne receiver



p 1955 J. STEGMUELLER MIXER STAGE OF A SUPERHETERODYNE RECEIVER FiledSept. 22, 1954 INVENTORI JOHANN-STEGMUELLER BY W 7M HIS ATTORNEY.

United States Patent MIXER STAG OF A SUPERHETERODYNE Ap li a Se ember 21 S al 5 60 Claims. (Cl. 250-40) My invention relates to the mixer stageof a superheterodyne receiver and particularly to a mixer circuitincluding'a selfoscillating mixer tube operating with additive mixing.Although it is particularly useful for ultrahigh-frequency, or UQH. F.,reception, it is not limited to use in this range." i

The use of a directly heated triode as a mixer tube in such a circuit,for'example in portable receivers, leads to great difiiculties inobtaining a sufliciently high oscillator voltage because the conversionconductance'of such mixer tubes is relatively small. Moreover, therequirement that stable operation of the oscillator be insured even inbattery operation and the resulting different voltages, must also befulfilled at the lower. voltage limit. It is' also desirable that thedifference in the mixer-amplifications should be as small as po ssible,whether new or old batteries are used.

My invention proposes a circuit which satisfies all these requirementsto a remarkably high degree. The invention comprises means forconnecting the oscillator circuit, in a capacitive three-point circuitbetween the three electrodes of the mixer tube in such a way that theintrmediatefrequency circuit, or at least the interme: diate frequencycoil, is decoupled from the plate of this mixer tubeat the oscillatorfrequency by a high frequency choke.

It is, therefore, an object of this invention to provide an improvedmixer stage for a superheterodyne receiver in which the intermediatefrequency circuit has little or no effect in the operation of theoscillator It is another object of the invention to provide an improvedmixer stage for a superheterodyne receiver in which the intermediatefrequencies produced in the mixer stage are prevented from appearing inthe source of signals.

It is still another object of this invention to provide an improvedmixer stage for a superheterodyne receiver in which the energy of theoscillator section is prevented from reaching the source.

The explanation of a high oscillator voltage obtained by the use of acapacitive three-point circuit is as follows: The tube capacitancesalready form a capacitive voltage divider. This is why it is notadvisable to use an inductive voltage divider for the three-pointcircuit, because when two voltage dividers are present at the same time,the optimum oscillatory. condition is unobtainable. To this is added thefact that the connections also play a part in the U. H. F. range and mayproduce undesirable phase changes. If, on the other hand, only the tubecapacitances are used as voltage dividers of the three-point circuit, orif additional capacitors are added to the tube capacitances (preferablyclose to the tube), only a single voltage divider, i. e., a capacitivevoltage divider, is obtained. i

In such an oscillatory circuit the. triode. alone will not produce themaximum oscillator voltage with self-oscillating mixer tube. It isnecessary to prevent a capacitive loading of the oscillator by theintermediate frequ n y r u bec u e t a dition p c an e r ulting from theaddition of the intermediate frequency coil reduces the resonanceimpedance of the oscillator circuit, not only because of the capacitanceitself, but mainly because the losses of this'capacitance are too highfor the U. H, F. oscillator circuit. This loading by the intermediatefrequency coil is avoided by the in.- sertion of a U. H. F. coil (21 M4choke).

Ac or to a fu ther de e pm a of he n ention, especially efiicient meansmay be used to eliminate the loading of the intermediate frequencycircuit without irn: pairing the working of the oscillator.

T e m n in wh h se v nta s resu may be obtained will be betterunderstood'after the following discussion of the single figure of thedrawing.

The resonant circuit tuned, for example, by means of a variablecapacitor is connected through the ca? pacitors C and C to the anode andgrid ofthe mixer tube R6. The three-point circuit is formed by the twocapacitors C1 and C2 and the parallel capacitance of the tube. In theline between the common point of ca: pacitqrs C1 and .Ca on one hand,and the ground on the other, is inserted the input circuit E so as toprevent the latter from carrying the oscillator voltage andradiat: ingthrough the antenna.

The intermediate frequency circuit Z 1, following the mixer tube, isessentially formed in this eXarnple by the f dhq h D2 and L and .h a asta h s Ci to q- Hf sha D re e t e paci anees s own to t r h in th d aram from b i h inested. n r lh it h ieih ah r uit, In h d hat theoscillator voltage may be applied mainly to the choke a. ca c o C s a deIt has be n und ha t b i e c r u t. orm d y h q a e ci i h the 1 12 1circuit 5. n th ia o i o i eq i br um ove h nt r r hei an e, u o h s tht he. soil Li i v se s a a itan which p n a y 1 11911 the frequency ofthe oscillator. The addition of the cap ci o C5 a s a c c l y fi ed aaci anc i qla h s nt me ia iie he c c c i Z1 rom the damping effects ofthe mixer tube, the applicant pro- P- F ap h e med a e re uency o a ean: pearing at point P, as the result of the action of a capacitiVflVoltage divider between the capacitors C1 and C to the grid of the mixertube via the grounded end of the input C Q i E- T s h v show hat he iruit Pro- POSed by h nt n. ui s, a. sli htl di eren me hod of removingthe damping. The capacitor C6 is made so. large that it presents ashort-circuit f;Qr he intermediate fre n so. that he n e m d a e t eieat point?- is. practically zero. The intermediate frequency voltage, u 0mines the damp i Obta ned. era ly by connecting. point P over U. H. E.choke D2 to the grid of the mixer. tube. The inductances L1 and D2,whose junc; tion point P- is grounded as far as; the intermediatefrequency is concerned, then form an inductive three-point circuit so.that an intermediatev frequency voltage for removing the damping isgenerated across the choke D2. This choke D also blocks the oscillatorvoltage, generated at the grid, from reaching point P, and thus fromground.

If the choke D2 is so designed that it will block the oscillatorfrequency, the intermediate frequency feedback, as was found by tests,may become too large. To avoid this, the capacitance C7 may be made sosmall that h ma ee of the w l D2 and. h c c tans Q added her w l se aermed a fre ncy o tage it the proper magnitude to be applied to thegrid,

The capacitive three-point circuit may also, be obtained by using 'acircuit proposed by applicant in which an.

inductance of the proper size is connected in parallel with thecapacitance on the plate side of the capacitive voltage 3 divider, inorder to obtain the optimum oscillatory conditions.

While I have illustrated a particular embodiment of my invention, itwill of course be understood that I do not wish to be limited theretosince various modifications both in the circuit arrangement and in theinstrumentalities may be made, and I contemplate by the appended claimsto cover any such modifications as fall within the true spirit and scopeof the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A mixer circuit for a superheterodyn receiver comprising, incombination, an oscillator amplifier having an anode, a grid and acathode, first and second capacitors connected in series between saidanode and said grid, a source of signal oscillations, connections forcoupling said source between the junction of said capacitors and saidcathode, a parallel resonant circuit for controlling the frequency ofoscillations of said amplifier, means for coupling said parallelresonant circuit between said anode and said grid, said capacitors andsaid parallel resonant circuit forming a bridge circuit that is balancedfor the frequency of the oscillations of said oscillator amplifier sothat energy from the oscillator is prevented from being coupled to saidsource.

2. A mixer circuit for a superheterodyne receiver comprising, incombination, an amplifier having an anode, a grid and a cathode, firstand second condensers coupled in series between said anode and saidgrid, an oscillatory circuit coupled between said anode and said grid, asource of signal oscillations coupled between the junction of said firstand second condensers and said cathode, said oscillatory circuit havinga high impedance for energy of an intermediate frequency that isproduced by the heterodyning of the signals from said source and theoscillation energy produced by said amplifier.

3. A mixer circuit for a superheterodyne receiver comprising, incombination, an oscillator amplifier having an anode, a grid and acathode, first and second capacitors connected in series between saidanode and said grid, a source of signals coupled between the junction ofsaid capacitors and said cathode, an oscillatory circuit coupled betweensaid anode and said cathode, said oscillatory circuit and saidcondensers forming a bridge circuit for energy of the frequency of saidoscillator in which said amplifier is connected in a first diagonal ofsaid bridge circuit and said source of signals is connected across asecond diagonal of said bridge circuit, said bridge circuit having apredetermined balance point such that energy of the oscillationfrequency appearing across said second diagonal and hence across saidsource is substantially zero.

4. A mixer circuit for a superheterodyne receiver comprising, incombination, an oscillator amplifier having an anode, a grid and acathode, first and second capacitors connected in series between saidanode and said grid, a source of signals coupled between the junction ofsaid capacitors and said cathode, an oscillatory circuit coupled betweensaid anode and said cathode, said oscillatory circuit and said condenserforming a bridge circuit for energy of the frequency of said oscillatorin which said amplifier is connected in a first diagonal of said bridgecircuit and said source of signals is connected across a second diagonalof said bridge circuit, said bridge circuit having a predeterminedbalance point such that energy of the oscillation frequency appearingacross said second diagonal and hence across said source is zero, atransformer having a primary and a secondary, means for coupling saidprimary between said anode and said grid in such manner that energy ofthe intermediate frequency appears at said anode and said grid withopposite respective phases.

5. A mixer circuit for a superheterodyne receiver comprising, incombination, an oscillator amplifier having an, anode, a grid and acathode, first and second capacitors connected in series between saidanode and said grid, a

source of signals coupled between the junction of said capacitors andsaid cathode, an oscillatory circuit coupled between said anode and saidcathode, said oscillatory circuit and said condensers forming a bridgecircuit for energy of the frequency of said oscillator in which saidamplifier is connected in a first diagonal of said bridge circuit andsaid source of signals is connected across a second diagonal of saidbridge circuit, said bridge circuit having a predetermined balance pointsuch that energy of the oscillation frequency appearing across saidsecond diagonal and hence across said source is substantially zero, atransformer having a primary and a secondary, means for coupling saidprimary winding between said anode and said grid, means for grounding anintermediate point of the circuit formed by said coupling means and saidprimary winding, the coupling means being of such nature that the ratioof the impedance for energy of intermediate frequency and theintermediate point and the impedance for intermediate frequency betweensaid grid and the intermediate point is the same as the ratio of theimpedance of said first and second condensers for energy of theintermediate frequency so that the bridge formed by said first andsecond condensers and said coupling means is balanced and energy of theintermediate frequency between the junction of said condensers and saidgrounded intermediate point has substantially zero amplitude, therebypreventing any energy of intermediate frequency from fiowing throughsaid source.

6. A mixer circuit for a superheterodyne receiver comprising, incombination, an amplifier having an anode, a grid and a cathode, firstand second condensers connected in series between said anode and saidgrid, an oscillatory circuit, means for coupling said oscillatorycircuit between said anode and said grid, a source of signals connectedbetween the junction of said first and second condensers and saidcathode, an intermediate frequency transformer having primary andsecondary windings, a choke coil for the frequency of said oscillatorycircuit connected between said anode and one end of said primarywinding, a third condenser connected between said cathode and the otherend of said primary winding, said third condenser providing a lowimpedance path for the intermediate frequency produced by the mixing ofthe signals from said source and the oscillations produced in saidamplifier under the control of said oscillatory circuit, and a phaseshifting impedance connected between said second end of said primarywinding and said grid so that energy of said intermediate frequency hassubstantially zero amplitude at the junction of said first and secondcondensers, thereby being prevented from passing through said source.

7. A mixer circuit as set forth in claim 6 wherein a fourth capacitorhaving a high impedance for frequencies in the range of the frequency ofsaid oscillatory circuit is connected between said cathode and thejunction of said choke and said primary winding so as to aid inpreventing detuning of said oscillatory circuit by said intermediatefrequency transformer.

8. A mixer circuit as set forth in claim 6 wherein said phase shiftingimpedance is comprised of achoke for intermediate frequencies.

9. A mixer circuit as set forth in claim 7 wherein said phase shiftingimpedance is comprised of a choke for intermediate frequencies and afifth condenser connected in series.

10. A mixer circuit for a superheterodyne receiver comprising anamplifier having an anode, a grid and a cathode, first and secondcondensers connected in series in the order named between said anode andsaid grid, a source of signals connected between the junction of thefirst and second condensers and said cathode, an oscillatory circuitconnected between said anode and said grid so as to cause said amplifierto oscillate at a frequency determined by said oscillatory circuit, saidoscillatory circuit having high impedance for the intermediate frequencyproduced by the beat between the frequency of said oscillator and thefrequency of said signal, an intermediate frequency transformer havingprimary and secondary windings, a choke coil for frequencies higher thanthe intermediate frequency connected between said anode and one end ofsaid primary winding, a third condenser having a low reactance forfrequencies of the oscillatory circuit and high reactance for theintermediate frequencies, means for connecting said third condenserbetween the junction of said choke and one end of said primary windingand said cathode, a fourth condenser having a low reactance for theintermediate frequency, means for connecting said fourth condenserbetween the other end of said winding and said cathode, a source offixed operating potential connected to said other end of said primarywinding, a choke coil and a fifth condenser connected in series so as toform a phase shifting impedance, said series combination being connectedbetween said other end of said primary winding and said grid so as toprovide at the junction of said first and second condensers a voltage ofintermediate frequency that is substantially equal and opposite to thevoltage of intermediate frequency arriving at said junction via saidfirst condenser, thus preventing any energy of the intermediatefrequency from passing through said source of signals.

No references cited.

